In converters represented by a power device which coverts input AC power to DC power and outputs a desired DC voltage, what is called an across-the-line capacitor (hereinafter referred to as X capacitor) that is a capacitor which reduces line noises is usually connected between AC voltage lines.
When a plug of the power device connected to the AC power supply is pulled out from a commercial power supply which is the AC power supply, electric charges accumulated until immediately before the pull-out remain in the X capacitor. At this time, there is a risk that these residual charges cause an electric shock to a person who touches an outlet portion of the plug just pulled out. There are safety standards for preventing such an electric shock, which demand that the X capacitor be discharged within a predetermined length of time when the AC power is shut off.
As a measure for this electric shock, a discharge resistor for allowing the residual charges to be discharged is conventionally provided in parallel with the X capacitor. However, this discharge resistor consumes power constantly while the power device is supplied with AC voltage. In particular, there is a problem that power consumed by the discharge resistor in the standby condition becomes too high to ignore.
In order to solve the above problem, Patent Literature (PTL) 1 discloses a power circuit which includes a transistor that switches the above discharge resistor between a conductive state and a non-conductive state and a control unit that controls the transistor between an ON state and an OFF state and in which, only when supplied with no AC voltage, the control unit turns on the transistor to form a discharging loop.